LESSON 6.2 — Sewerage and Solid Waste Management
§A — Sewage Generation
§A.1 The 80% Rule
Sewage is not a separate source — it is directly derived from water supply. The CPHEEO Manual establishes the standard design rule:
Sewage generation = 80% of water supplied
The remaining 20% is accounted for by evapotranspiration, garden/irrigation use, car washing, and water retained in products and processes — water that enters the system but does not return as liquid wastewater to the sewer.
Design implication: The Sewage Treatment Plant (STP) capacity is sized on the sewage volume, not the water demand. Sizing the STP at 80% of the water demand is both the planning norm and a GATE numerical standard.
Sewage generation norms by settlement type:
| Settlement Type | Water Supply Norm (lpcd) | Sewage Generation (lpcd) | Basis |
|---|---|---|---|
| Town — no sewerage | 70 | 56 | 80% of 70 |
| City — with sewerage | 135 | 108 | 80% of 135 |
| Metro / Mega city | 150 | 120 | 80% of 150 |
Note: Where sewerage is absent (70 lpcd towns), sewage does not enter a centralised sewer network. The 80% rule is applied when designing sewers for cities where sewerage exists or is planned.
Peaking factors for sewer design (CPHEEO):
| Flow Type | Factor × Average Dry Weather Flow | Use |
|---|---|---|
| Average Dry Weather Flow (ADWF) | 1.0 | Treatment plant design baseline |
| Peak hourly flow | 3.0 × ADWF | Sewer pipe capacity design |
| Storm flow (combined sewer) | 6 × ADWF | Combined sewer overflow design |
| Minimum flow (minimum daily) | 1/3 × ADWF | Self-cleansing velocity check |
Self-cleansing velocity: Sewers must maintain a minimum flow velocity of 0.6–0.9 m/s to prevent sedimentation of suspended solids. Sewers are designed on a gradient that achieves this velocity at minimum flow.
§B — Sewerage System Types
Three distinct sewerage configurations exist, each with different treatment cost, capital cost, and hydraulic loading implications.
§B.1 System Classification
| System | What It Carries | Network Structure | Relative Capital Cost | Treatment Cost |
|---|---|---|---|---|
| Separate | Foul water (sewage) in dedicated foul sewer; stormwater in dedicated storm drain | Two independent pipe networks | Higher (two networks) | Lower — only foul water treated |
| Combined | Both sewage and stormwater in a single pipe to one sewer | Single pipe network | Lower (one network) | Higher — entire combined volume (including stormwater) must be treated |
| Partially Separate | Most stormwater to storm drain; limited rooftop/clean stormwater permitted into foul sewer to prevent local flooding during heavy rainfall | Modified separate system; controlled cross-connections | Intermediate | Intermediate |
§B.2 Selection Logic
| Condition | Recommended System | Reason |
|---|---|---|
| New township; both foul and storm sewers planned | Separate | Avoids treating clean stormwater; reduces STP load and cost |
| High-rainfall city with existing combined sewer legacy | Combined (existing) or partial upgrade | Retrofitting separate system in built urban area is prohibitively expensive |
| Area prone to local flooding from roof runoff choking storm drains | Partially separate | Controlled diversion of clean roof runoff into foul sewer relieves storm drain; minimal treatment impact |
| Older Indian cities (pre-1960s planned areas) | Often combined | Built before separate system became standard; retrofitting typically not feasible |
| Industrial areas generating chemical/trade effluent | Mandatory separate | Industrial effluent requires pre-treatment before mixing with domestic sewage; must not enter storm drain |
GATE MCQ pattern: “A new residential colony in Pune has both foul sewer and storm drain available. Which system should be adopted?” → Separate system. Reason: avoids treating stormwater; reduces STP capacity and cost; both networks already available from the municipality.
§B.3 Sewer Appurtenances
Key infrastructure elements associated with sewerage systems:
| Element | Function | Key Standard |
|---|---|---|
| Manhole | Access point for inspection, cleaning, and repair; placed at junctions, changes of direction, gradient changes | Max spacing: 90 m on straight runs; 45 m on curved sections |
| Intercepting trap | Deep-seal trap (100 mm) at the last inspection chamber before the public sewer; prevents sewer gases from entering private drains | IS 5329 |
| Flushing tank | Automatic siphon-fed tank that periodically flushes the upstream end of sewers to maintain self-cleansing | At dead-ends and flat-gradient sections |
| Inverted siphon (depressed sewer) | Where sewer must pass below an obstacle (road, river, utility); runs full under pressure rather than by gravity | Requires minimum velocity 0.9 m/s |
| Pumping station (lift station) | Raises sewage where gravity flow is impossible; required in flat terrain or where sewer depth becomes impractical | Typically wet-well/dry-well design |
§C — BOD and COD
§C.1 Definitions
These two parameters are the primary measures of organic pollution load in wastewater. Both are expressed in mg/L (milligrams of oxygen per litre of water sample).
BOD — Biochemical Oxygen Demand:
The quantity of dissolved oxygen consumed by aerobic microorganisms to biodegrade the biodegradable organic matter in a water sample, measured over 5 days at 20°C (standard test: BOD₅). It represents the oxygen demand of the living biological fraction of organic pollution.
COD — Chemical Oxygen Demand:
The quantity of oxygen required to chemically oxidise all organic matter in a water sample (both biodegradable and non-biodegradable) using a strong oxidising agent (typically potassium dichromate). COD is faster to measure (2–3 hours vs. 5 days for BOD) and always accounts for a superset of what BOD measures.
Fundamental relationship: COD ≥ BOD for any water sample. The gap between COD and BOD (COD − BOD) represents the non-biodegradable organic fraction.
§C.2 BOD vs. COD — Comparison Table
| Feature | BOD | COD |
|---|---|---|
| Full form | Biochemical Oxygen Demand | Chemical Oxygen Demand |
| What it measures | Biodegradable organic matter only | All organic matter (biodegradable + non-biodegradable) |
| Test method | Biological (aerobic microbes, 5 days, 20°C) | Chemical oxidation (K₂Cr₂O₇ in acid; 2–3 hours) |
| Test duration | 5 days (BOD₅) | 2–3 hours |
| Typical unit | mg/L (as O₂) | mg/L (as O₂) |
| Relationship | Always ≤ COD | Always ≥ BOD |
| Use in design | Biological treatment sizing; effluent discharge compliance | Industrial wastewater characterisation; faster process monitoring |
| Raw domestic sewage (typical) | 200–350 mg/L | 400–600 mg/L |
| Good secondary effluent | < 30 mg/L | < 90 mg/L |
| After tertiary treatment | < 10 mg/L | < 30 mg/L |
| Discharge standard (CPCB into inland surface water) | ≤ 30 mg/L | ≤ 250 mg/L |
Exam anchor — the inequality trap: “BOD is always greater than COD for the same sample” — FALSE. COD ≥ BOD, always. A sample can have BOD = COD only if all organic matter is biodegradable (theoretical; rare in practice).
§C.3 Typical Effluent Quality Targets (Indian Standards)
| Discharge Destination | BOD Limit | COD Limit | TSS Limit | Source |
|---|---|---|---|---|
| Inland surface water | ≤ 30 mg/L | ≤ 250 mg/L | ≤ 100 mg/L | CPCB |
| Land irrigation | ≤ 100 mg/L | — | ≤ 200 mg/L | CPCB |
| Marine coastal discharge | ≤ 100 mg/L | — | ≤ 100 mg/L | CPCB |
| Potable reuse (tertiary treated) | < 10 mg/L | < 30 mg/L | < 10 mg/L | WHO / MoHUA |
§D — Sewage Treatment Stages
§D.1 Full Treatment Sequence
Raw Sewage Inlet
↓
[0] PRELIMINARY — Screening + Grit Removal
↓
[1] PRIMARY — Sedimentation (Clarification)
↓
[2] SECONDARY — Biological Treatment
↓
[3] TERTIARY — Disinfection + Nutrient Removal + Polishing
↓
Effluent Discharge / Reuse
↓
[Sludge Treatment] — Thickening → Digestion → Dewatering → Disposal
§D.2 Stage-by-Stage Detail
Stage 0 — Preliminary Treatment
- Screening: Bar screens / fine screens remove large solids (rags, plastic, leaves) that would damage downstream equipment.
- Grit removal: Grit channels or grit chambers allow inorganic sand, gravel, and grit to settle; prevents abrasion of pumps and pipes.
- What it removes: Physical solids > 6 mm; inorganic grit.
- BOD removal: Negligible.
Stage 1 — Primary Treatment (Sedimentation)
- Process: Screened and degritted sewage flows into a primary settling tank (clarifier); residence time ~2 hours.
- Mechanism: Gravity settling of suspended organic solids as primary sludge.
- Removes: 50–70% of suspended solids; 25–40% of BOD.
- Output: Settled effluent (primary effluent) → Secondary treatment; Primary sludge → Sludge treatment.
Stage 2 — Secondary Treatment (Biological)
Secondary treatment is the critical stage — it removes dissolved and colloidal organic matter through biological activity. Multiple technology options exist (see §D.3 below).
- Removes: 85–95% of BOD overall (from raw influent); suspended solids to 30–50 mg/L.
- Output: Secondary effluent (BOD typically 20–50 mg/L) → Tertiary treatment or direct discharge if within limits.
Stage 3 — Tertiary Treatment (Advanced Polishing)
- Disinfection: Chlorination (Cl₂), UV irradiation, or ozonation to kill residual pathogens.
- Nutrient removal: Nitrogen and phosphorus removed by additional biological or chemical processes (critical for discharge to lakes and coastal waters to prevent eutrophication).
- Filtration: Fine filtration (sand or membrane) to reduce residual TSS.
- Removes: Residual BOD to < 10 mg/L; pathogens to near-zero; nutrients.
- Output: Tertiary effluent suitable for non-potable reuse (irrigation, cooling, toilet flushing) or stringent discharge.
§D.3 Secondary Treatment Technologies (MoUD Category Framework)
MoUD/MoHUA (2012) Technology Categories:
| Category | Performance | Energy | Land | Representative Technologies |
|---|---|---|---|---|
| I | BOD < 30, SS < 30 | Low | High | DEWATS, WSPS, Duckweed Pond, FAL |
| II | BOD < 30, SS < 30 | High | Low-Moderate | ASP, CASP, UASB, BIOFOR |
| II (Improved) | BOD < 20, SS < 20 | Very high | Low | Advanced ASP with nitrification |
| III | BOD < 30, SS < 30 | Moderate | Moderate | Trickling filter, rotating biological contractor |
| IV | BOD < 30, SS < 30 | Low | Moderate-low | Constructed wetlands, waste stabilization ponds (marginal) |
Key technologies at a glance:
| Technology | Type | BOD Removal | Energy Need | Key Feature / Constraint |
|---|---|---|---|---|
| ASP (Activated Sludge) | Aerobic, suspended growth | 85–95% | High (aeration) | Most common in Indian cities; uninterrupted power essential |
| CASP (Cyclic ASP) | Aerobic, batch | Up to 98%; BOD < 10 mg/L | High (aeration) | Aeration + settling in same tank; compact; urban infill sites |
| UASB | Anaerobic, upflow | BOD → 70–100 mg/L (partial) | Nil (no external energy) | Produces biogas; effluent cannot be directly discharged; requires polishing |
| BIOFOR | Attached growth, biological filtration | 94–99.9% overall | High (chemical + aeration) | Biolite media; very low land; two-stage filtration |
| Trickling Filter | Attached growth, aerobic | 50–95% (rate dependent) | Low | Simple; low O&M; biofilm on media; secondary clarifier required |
| WSPS (Waste Stabilization Pond) | Natural, solar-powered | > 90% BOD; 6-log coliform reduction | None | High land; suitable for warm climates; aquaculture reuse possible |
| DEWATS | Decentralised, multi-stage | Up to Cat I performance | Low | 4-stage process; for small communities/institutions without central sewer |
| FAL (Facultative Aerated Lagoon) | Natural/oxidation pond | 70–90% BOD | None (natural aeration) | Detention time ≥ 3 days; depth 2–5 m; low cost |
UASB critical trap: UASB effluent BOD of 70–100 mg/L exceeds the 30 mg/L discharge standard for inland waters. UASB is a primary + partial secondary treatment technology, not a standalone secondary treatment. Post-treatment (polishing pond or constructed wetland) is mandatory.
§E — Solid Waste Management
§E.1 Definition and Generation Rates
Solid waste is any discarded material that is not a liquid or gas — the residual of all consumption and production activities. It is categorised, collected, processed, and disposed under the framework of the SWM Rules, 2016 (MoEFCC, under the Environment Protection Act, 1986).
Per capita generation (URDPFI 2014):
| Land Use | Generation Rate |
|---|---|
| Residential | 0.3–0.6 kg/capita/day |
| Commercial | 0.1–0.2 kg/capita/day |
| Street sweepings | 0.05–0.2 kg/capita/day |
| Institutional | 0.05–0.2 kg/capita/day |
India total urban MSW: approximately 1,50,000 tonnes/day (CPCB 2020–21). Growth rate: ~1.3%/year.
§E.2 Waste Categories
A. Municipal Solid Waste (MSW)
Mixed domestic, commercial, and institutional waste from urban areas. Composition varies significantly by income level and city size:
| Component | High-income city (% by weight) | Low-income city (% by weight) |
|---|---|---|
| Organic / food waste | 35–45% | 55–65% |
| Paper & cardboard | 10–15% | 5–8% |
| Plastic | 8–12% | 5–8% |
| Metal | 2–4% | 1–2% |
| Glass | 2–4% | 1–2% |
| Inert (soil, sand, ash) | 10–15% | 15–25% |
Indian MSW has high organic content and high moisture — directly influences treatment technology selection (favours composting and bio-methanation over incineration in most cases).
B. Biomedical Waste (BMW)
Generated during the diagnosis, treatment, immunisation, or research activities in health facilities.
- Regulated by: Bio-Medical Waste Management Rules, 2016 (MoEFCC)
- Key categories: Sharps (needles, scalpels), soiled dressings and bandages, anatomical waste (body parts, placenta), cultures and laboratory waste, discarded medicines, liquid waste from patients, chemical waste
- Treatment: Autoclaving (steam sterilisation), microwave treatment, incineration in dedicated BMW incinerators; deep burial for anatomical waste in isolated locations
- Colour coding: Yellow (incinerable), Red (autoclavable/microwaveable), White (sharps), Blue (glass)
- Key restriction: BMW must not be mixed with municipal waste at any point
C. Hazardous Waste
Waste exhibiting one or more of: ignitability, corrosivity, reactivity, or toxicity (ICRT classification). Sources: industrial manufacturing, paint, batteries, solvents, pesticides, electroplating sludge.
- Regulated by: Hazardous Waste Management Rules, 2016
- Disposal: Secure landfill (lined, leachate-collected, monitored), incineration at high-temperature dedicated incinerators, chemical stabilisation/solidification before landfill
- Extended Producer Responsibility (EPR): Manufacturers responsible for take-back and safe disposal
D. E-Waste (Electronic Waste)
Discarded electronic and electrical equipment — computers, phones, televisions, refrigerators, washing machines.
- Regulated by: E-Waste Management Rules, 2016 (amended 2022)
- Composition: Contains recoverable metals (gold, silver, palladium, copper, rare earth elements) and hazardous materials (lead, mercury, cadmium, hexavalent chromium, brominated flame retardants)
- Key requirement: EPR mandates manufacturers and importers to register with CPCB and ensure collection and recycling through authorised dismantlers
- Current issue: ~90% of Indian e-waste is processed by the informal sector under hazardous conditions without proper containment of toxins
E. Construction & Demolition (C&D) Waste
- Regulated by: C&D Waste Management Rules, 2016
- Recycling pathway: Concrete → recycled aggregate; masonry → manufactured sand and blocks; steel → scrap recycling; C&D waste should be processed at dedicated processing facilities before landfill
§E.3 SWM Process Flow
Generation
↓
Segregation at Source (Green / White / Black bins)
↓
Primary Collection (door-to-door; handcart / vehicle)
↓
Secondary Collection (community bins / transfer stations)
↓
Transportation (covered vehicles; no leachate spillage)
↓
Processing / Treatment
↓
Disposal (residuals to landfill)
Colour-coded bin system (SWM Rules, 2016):
| Bin Colour | Waste Type | Examples |
|---|---|---|
| Green | Biodegradable / wet | Food waste, vegetable peels, garden/yard waste |
| White / Blue | Dry / recyclable | Paper, cardboard, plastic, glass, metal |
| Black | Domestic hazardous | Sanitary napkins, diapers, e-waste, batteries, expired medicines |
Kabadi system: The informal waste recycling network handles an estimated 70–75% of all recyclables in India through waste pickers and itinerant buyers. SWM Rules, 2016 direct ULBs to integrate informal waste workers into formal systems.
§F — SWM Hierarchy (ISWM Framework)
The Integrated Solid Waste Management (ISWM) hierarchy defines a priority order for managing waste — moving from the most preferred (generation prevention) to the least preferred (disposal). This hierarchy is embedded in SWM Rules, 2016 and aligned with EU and WHO frameworks.
§F.1 5-Level Hierarchy
1. REDUCTION (Source reduction / Prevention) ← Most preferred
↓
2. REUSE
↓
3. RECYCLING
↓
4. RECOVERY (Energy / material recovery)
↓
5. DISPOSAL (Landfill) ← Least preferred
| Level | Definition | Examples |
|---|---|---|
| 1. Reduction | Prevent waste from being generated in the first place; reduce consumption of materials | Lightweight packaging; designing for durability; bulk buying to reduce packaging |
| 2. Reuse | Use the product again without reprocessing | Refillable bottles; second-hand furniture; repaired electronics |
| 3. Recycling | Transform waste into a new product or raw material for new products | Paper pulp from wastepaper; plastic granules from PET bottles; steel scrap smelting |
| 4. Recovery | Extract energy or materials from waste that cannot be reused or recycled | Incineration with heat recovery; biogas from bio-methanation; pyrolysis oils |
| 5. Disposal | Final management of residual waste that has no further use or recovery potential | Sanitary landfill; secure landfill for hazardous residue |
GATE MCQ trap: “Which of the following is the MOST preferred option in the ISWM hierarchy?” → Source Reduction / Prevention, not recycling. Recycling is 3rd, after prevention (1st) and reuse (2nd).
§G — Disposal Options: Landfill, Composting, Incineration
§G.1 Comparative Overview
| Option | Best Condition | Feedstock Requirement | Output | Key Constraint |
|---|---|---|---|---|
| Sanitary Landfill | Residual inert/non-processable waste after all other options exhausted | Any non-liquid, non-hazardous solid waste (residuals) | Gas (methane — collect or flare); leachate (treat) | Land scarcity; NIMBY; leachate contamination risk; 20–25 year life |
| Composting | High organic content waste; moisture 50–60% | Segregated biodegradable waste (food, garden) — no plastics or hazardous | Compost (soil conditioner); no energy | Requires reliable source segregation; odour management; market for compost |
| Bio-methanation | High organic, high moisture waste (typical Indian MSW profile) | Segregated wet organic waste; minimum 60–65% organic content | Biogas (CH₄ + CO₂) → energy; digestate (fertiliser) | Requires enclosed digester; gas utilisation infrastructure; skilled O&M |
| Incineration | Low-moisture, high-calorific waste (minimum 1,500 kcal/kg) | Dry waste; low-organic fraction; excludes chlorinated plastics | Ash (~10% of original volume); heat/electricity; flue gas | High capital cost; air emission controls required; not suitable for wet Indian MSW without pre-drying |
| Pyrolysis | Plastic-rich waste stream; mixed non-recyclable fraction | Dry, plastic-dominant or mixed dry waste | Char + liquid oils + combustible gas (3 output streams) | Endothermic; requires external heat input; no oxygen; complex product handling |
| Gasification | Refuse-derived fuel (RDF); dried MSW | High-calorific, low-moisture feedstock | Syngas (CO + H₂ + CH₄) + ash | Partial oxygen; exothermic; syngas can be used for power generation |
§G.2 Sanitary Landfill — Design Components
Active life: 20–25 years. Post-closure monitoring: 30+ years.
5 essential design components (IS 12647:1999 / SWM Rules 2016):
| Component | Function |
|---|---|
| 1. Liner system | HDPE geomembrane + RCC base; prevents leachate and landfill gas from migrating into surrounding soil and groundwater |
| 2. Leachate collection & treatment | Perforated pipe network at base collects leachate (toxic liquid from percolating rainwater); conveyed to leachate treatment plant |
| 3. Gas collection & control | Vertical gas wells and horizontal pipes collect landfill gas (CH₄ + CO₂ from anaerobic decomposition); flared or used for energy recovery |
| 4. Final cover system | Low-permeability soil + vegetative layer placed on completed fill; prevents rainwater infiltration; supports surface revegetation |
| 5. Surface water drainage | Channels and berms divert all rainfall runoff away from the fill area; reduces leachate generation and slope erosion |
Leachate is the most significant environmental risk: rainwater percolating through the waste mass dissolves heavy metals, ammonia, chlorinated compounds, and pathogens. If the liner fails, leachate contaminates groundwater.
§G.3 Composting — Process Detail
| Stage | Temperature | Duration | Process |
|---|---|---|---|
| Mesophilic | 20–45°C | Days 1–3 | Initial aerobic decomposition; rapid temperature rise |
| Thermophilic | 50–70°C | Days 4–14 | Rapid decomposition; pathogens destroyed above 55°C; weed seeds eliminated above 62°C |
| Cooling / Maturation | 30–40°C | Days 15–30+ | Stabilisation; humus formation; compost matures |
Three composting methods:
| Method | Description | Scale |
|---|---|---|
| Windrow (manual/mechanical) | Elongated piles turned periodically for aeration | Large open sites; municipal scale |
| In-vessel (mechanical) | Enclosed rotating drum or container; controlled temperature, aeration, moisture | Compact sites; controlled environment; faster cycle |
| Vermicomposting | Earthworms (Eisenia fetida) process organic matter; produces worm castings | Decentralised; household to community scale; high-quality output |
Temperature trap: “Composting sterilises all biological agents at 50°C” — FALSE. Pathogens die above 55°C; weed seeds require 62°C. A question about pathogen destruction and weed seed elimination have different temperature thresholds.
§D (Required Section) — BOD/COD Table + Sewage Volume Worked Example
D.1 BOD vs. COD Master Comparison
| Attribute | BOD₅ | COD |
|---|---|---|
| Measures | Biodegradable organics only | All organics (biodegradable + non-biodegradable) |
| Test duration | 5 days at 20°C | 2–3 hours |
| Reagent | Dissolved oxygen (biological) | K₂Cr₂O₇ in H₂SO₄ (chemical) |
| Always true | BOD ≤ COD | COD ≥ BOD |
| Raw domestic sewage | 200–350 mg/L | 400–600 mg/L |
| After primary treatment | 120–250 mg/L | 250–450 mg/L |
| After secondary treatment (good ASP) | 20–30 mg/L | 60–90 mg/L |
| CPCB discharge limit (inland water) | ≤ 30 mg/L | ≤ 250 mg/L |
| Design purpose | Biological reactor sizing; effluent compliance | Industrial wastewater characterisation; quick process control |
D.2 Worked Numerical — Sewage Volume (80% Rule)
Problem: A city has a population of 12 lakh. The city has a fully operational sewerage system. Calculate:
(a) Average Daily Water Demand (ADD) in MLD
(b) Average daily sewage generation in MLD
(c) Peak hourly sewage flow in MLD (for sewer pipe design)
(d) STP design capacity in MLD (using ADD of sewage with a 25% contingency allowance)
Solution:
(a) Average Daily Water Demand:
Per capita norm = 135 lpcd (city with sewerage, URDPFI 2014)
ADD = Population × lpcd = 12,00,000 × 135 L = 1,62,00,00,000 L/day
ADD = 162 MLD
(b) Average Daily Sewage Generation:
Sewage = 80% of water supply = 0.80 × 162 = 129.6 MLD
(c) Peak Hourly Sewage Flow:
Peak hourly factor = 3.0 × Average Dry Weather Flow (ADWF) = 3 × ADWF
Note: ADWF ≈ Average daily sewage flow / 24 hours (for per-hour calculation)
Daily average = 129.6 MLD = 129.6 × 10⁶ L/day
Peak hourly flow = 3.0 × 129.6 = 388.8 MLD (expressed as equivalent daily rate)
(This is the instantaneous peak demand for sewer pipe sizing — actual peak flow = 3× the average daily rate expressed on a per-day basis.)
(d) STP design capacity with 25% contingency:
STP capacity = 129.6 × 1.25 = 162 MLD
Answer summary:
– ADD (water) = 162 MLD
– Sewage = 129.6 MLD
– Peak hourly sewer design flow = 388.8 MLD equivalent
– STP design capacity = 162 MLD
D.3 Worked Numerical — BOD Load Calculation
Problem: The 12-lakh city in D.2 discharges sewage with an influent BOD of 280 mg/L. After secondary treatment (ASP), the effluent BOD is 28 mg/L. Calculate:
(a) Daily BOD load entering the STP (kg/day)
(b) Daily BOD load in the treated effluent (kg/day)
(c) BOD removal efficiency (%)
Solution:
(a) Influent BOD load:
Daily sewage flow = 129.6 MLD = 129.6 × 10⁶ L/day = 1.296 × 10⁸ L/day
BOD load (kg/day) = Flow (L/day) × BOD concentration (mg/L) ÷ 10⁶
= 1.296 × 10⁸ × 280 ÷ 10⁶ = 36,288 kg/day ≈ 36.3 tonnes/day
(b) Effluent BOD load:
= 1.296 × 10⁸ × 28 ÷ 10⁶ = 3,628.8 kg/day ≈ 3.6 tonnes/day
(c) BOD removal efficiency:
= (36,288 − 3,629) / 36,288 × 100 = 32,659 / 36,288 × 100 = 90%
90% BOD removal is consistent with a well-operated secondary treatment plant (ASP achieving 85–95% BOD removal per CPHEEO norms).
§H — Mini-Check: Practice Questions
MSQ 1 — BOD/COD
Which of the following statements are correct? (Select all that apply)
(A) BOD is measured over 5 days at 20°C using dissolved oxygen consumption by microorganisms.
(B) COD is always less than BOD for the same water sample.
(C) A sample with BOD = 250 mg/L could have a COD of 400 mg/L.
(D) BOD measures only biodegradable organic matter; COD measures all organic matter.
(E) COD provides a faster measurement than BOD.
Answer: A, C, D, E
Rationale:
– (A) Correct — standard BOD₅ definition.
– (B) Wrong — COD ≥ BOD always; this statement reverses the inequality.
– (C) Correct — COD (400) ≥ BOD (250); this is a valid scenario.
– (D) Correct — the definitional distinction.
– (E) Correct — COD test: 2–3 hours; BOD₅: 5 days.
MSQ 2 — Sewerage System Selection
A city planning authority is reviewing sewerage systems for three different scenarios. Which system should be selected for each? (Match or select)
(A) New satellite township; municipality provides both a foul sewer and a storm drain in the trunk infrastructure → Separate system
(B) An old industrial zone with chemical effluent discharges from multiple factories → Separate system (industrial effluent must not enter storm drain or mix without pre-treatment)
(C) A high-rainfall coastal city with an existing combined sewer where retrofitting is economically infeasible → Combined (retain existing)
(D) A low-lying residential area where heavy monsoon runoff chokes the storm drain and causes basement flooding; a modified connection to the foul sewer from selected rooftop drains is proposed → Partially separate
All four correct answers stated above.
MCQ 1 — SWM Hierarchy Order
Arrange the following options in the correct ISWM hierarchy order from most preferred to least preferred:
(i) Sanitary landfill
(ii) Recycling
(iii) Source reduction / Prevention
(iv) Reuse
(v) Energy recovery
Correct order: (iii) → (iv) → (ii) → (v) → (i)
Which of the following options presents this order correctly?
(A) Reuse → Reduction → Recycling → Recovery → Landfill
(B) Reduction → Reuse → Recycling → Recovery → Landfill
(C) Recycling → Reduction → Reuse → Recovery → Landfill
(D) Reduction → Recycling → Reuse → Recovery → Landfill
Answer: (B)
MCQ 2 — Secondary Treatment Technology
Which of the following sewage treatment technologies can achieve BOD removal up to 98%, operates in a batch cycle in a single tank (combined aeration and settling), and is specifically suited to congested urban infill sites?
(A) ASP (Activated Sludge Process)
(B) UASB Reactor
(C) CASP (Cyclic Activated Sludge Process)
(D) Trickling Filter
Answer: (C)
Rationale: CASP = Cyclic (batch) ASP; same tank for aeration + settling; 98% BOD removal; compact footprint. ASP requires a separate secondary clarifier. UASB is anaerobic and achieves only partial BOD removal (effluent ~70–100 mg/L). Trickling filter is attached-growth with 50–95% BOD removal.
NAT 3 — Sewage Volume (80% Rule)
A city with piped water supply and sewerage has a population of 5 lakh. Per capita water supply norm = 135 lpcd. Calculate daily sewage generation in MLD using the standard CPHEEO rule.
Answer: 54 MLD
Working:
Water supply = 5,00,000 × 135 L/day = 67,500,000 L/day = 67.5 MLD
Sewage = 80% × 67.5 = 54 MLD
§I — Exam Traps & Anchors
| Trap | Correct Answer |
|---|---|
| “COD < BOD for the same sample” | False — COD ≥ BOD always |
| “UASB is complete secondary treatment” | False — UASB achieves only partial BOD removal (70–100 mg/L); requires post-treatment polishing before discharge |
| “Composting requires no source segregation” | False — composting requires segregated organic waste; plastics, glass, and hazardous waste contaminate the process |
| “Incineration is best for wet Indian MSW” | False — Indian MSW has high moisture and high organic content; incineration requires minimum 1,500 kcal/kg calorific value; wet waste needs pre-drying, making it costly |
| “Landfill is the preferred disposal method” | False — landfill is the last resort in the ISWM hierarchy; preferred order is Reduction → Reuse → Recycling → Recovery → Landfill |
| “Sewage volume = water supply volume” | False — sewage = 80% of water supplied; 20% is consumed/evaporated and does not return as sewage |
| “Pyrolysis needs partial oxygen” | False — pyrolysis = zero oxygen, endothermic; gasification = partial oxygen, exothermic |
| “Pathogens in compost are eliminated at 50°C” | False — pathogens require > 55°C; weed seeds require > 62°C |
| “Combined sewer is always better because it is cheaper” | Context-dependent — lower capital cost but higher treatment cost; combined sewer overflows (CSOs) can release untreated sewage during storms. Separate is preferred for new planned development |
| “SWM bin: black = recyclable” | False — Black = hazardous/domestic hazardous; White/Blue = recyclable; Green = biodegradable |